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Old 2013-02-21, 20:10   #1
Flatlander
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Default i7-2600k on Windows 8.

Why does Prime95 choose 4 threads instead of 8?

Would switching to 64bit Windows give me a speed increase?

(I haven't used Prime95 in ages.)


Last fiddled with by Flatlander on 2013-02-21 at 20:14 Reason: typos, typos, typos
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Old 2013-02-21, 20:34   #2
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Quote:
Originally Posted by Flatlander View Post
Would switching to 64bit Windows give me a speed increase?
from another thread:
Quote:
Originally Posted by Prime95 View Post
BTW, if your CPU supports 64-bit then 64-bit prime95 is 10-15% faster than 32-bit prime95.
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Old 2013-02-21, 21:21   #3
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Thanks. I'll re-install tonight when everything hits 100%. I assume the save files won't transfer?
64bit will run hotter?
Should I force 8 threads?
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Old 2013-02-21, 21:23   #4
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The save files should transfer. The intermediate data is independent of calculation method.
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Old 2013-02-21, 21:48   #5
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Quote:
Originally Posted by Flatlander View Post
Why does Prime95 choose 4 threads instead of 8?


1. Hyperthreading is turned off in the bios.
2. P95 didn't detect all cpu configs.
3. you can manually set it to 8 from the menu.


Expect to see ~0.026 iteration times for 4 threads and ~0.050 for 8 threads as hyperthreading comes at a cost. (4.0 ghz, dual channel memory, LL testing).

Your mileage may vary a bit, but you'll see similar results.

Last fiddled with by swl551 on 2013-02-21 at 21:56
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Old 2013-02-21, 22:02   #6
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Quote:
Originally Posted by swl551 View Post
1. Hyperthreading is turned off in the bios.
2. P95 didn't detect all cpu configs.
3. you can manually set it to 8 from the menu.


Expect to see ~0.026 iteration times for 4 threads and ~0.50 for 8 threads as hyperthreading comes at a cost. (4.0 ghz, dual channel memory, LL testing).

Your mileage may vary a bit, but you'll see similar results.
I'm getting 0.030, about right.
Quote:
[Main thread Feb 20 19:18] Mersenne number primality test program version 27.7
[Main thread Feb 20 19:18] Optimizing for CPU architecture: Core i3/i5/i7, L2 cache size: 256 KB, L3 cache size: 8 MB
[Main thread Feb 20 19:18] Logical CPUs 1,5 form one physical CPU.
[Main thread Feb 20 19:18] Logical CPUs 2,6 form one physical CPU.
[Main thread Feb 20 19:18] Logical CPUs 3,7 form one physical CPU.
[Main thread Feb 20 19:18] Logical CPUs 4,8 form one physical CPU.
[Main thread Feb 20 19:18] Starting workers.
Quote:
Core(TM) i7-2600K CPU @ 3.40GHz
CPU speed 3392.28 MHz, 4 hyperthreaded cores
CPU features: Prefetch SSE, SSe2 SSe4, AVX
...
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Old 2013-02-21, 22:17   #7
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Quote:
Originally Posted by Flatlander View Post
I'm getting 0.030, about right.
Looks good. However you on version 27.7 and 27.9 is available. I have read posts where people stated better stability with hyperthreading using 27.9. I never had a problem with 27.7, but now run all my PCs on 27.9 anyway.

Last fiddled with by swl551 on 2013-02-21 at 22:49
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Old 2013-02-22, 00:27   #8
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4. Hyper-threading makes it actually slower, at least for LL tests.
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Old 2013-02-23, 20:56   #9
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Thanks. Things are starting to make sense again.
But, what happened to P90 years!? (Or was it days?)
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Old 2013-02-24, 19:03   #10
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From the 'Best way to find large factors thread',
Quote:
Originally Posted by ATH View Post
P-1 will also run in around 1 day (anywhere from 12 hours to 2-3 days probably) depending on how much RAM you give it and how fast your CPU is.

It's almost a "waste" now to trialfactor on a CPU since GPU's are so much faster at it and P-1 is more needed atm, but it is of course up to you what you want to do.

For P-1 you should give at least 300 Mb RAM for every thread that runs P-1, and the more the better.
1) If I choose P-1 work should I run on 8 threads or 4?
(2560K of 4096K reserved 24/7, i7-2600k on 64bitWin8)

2) If 8 threads is faster is that enough memory? Candidates are at 62-63M.
Stage 1 of M62,089,483 says,
Quote:
Using AVX FFT length 3360K, ...
Thanks.
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Old 2013-02-24, 19:13   #11
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Quote:
Originally Posted by Flatlander View Post
From the 'Best way to find large factors thread',

1) If I choose P-1 work should I run on 8 threads or 4?
(2560K of 4096K reserved 24/7, i7-2600k on 64bitWin8)

2) If 8 threads is faster is that enough memory? Candidates are at 62-63M.
Stage 1 of M62,089,483 says,

Thanks.
I would say 4 is probably better -- the more independent threads, the more memory bandwidth you need, and with AVX and DDR3-1333 you're pretty close the memory-bottlenecking limit as it is.

(Testing of course is the best way to produce an answer )
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